Longevity in multi-cellular organisms is dependent upon tissue homeostasis, which in turn depends on the persistence of stem cell populations that are quiescent, but retain the long-term capacity to re-enter the cell cycle to self-renew, or to produce progeny that can differentiate and re-populate the tissue. Deregulated release of these cells from the quiescent state, or preventing them from entering quiescence, results in uncontrolled proliferation and cancer. Conversely, loss of quiescent cells, or their failure to re-enter cell division, disrupts organ development and prevents tissue regeneration and repair in aging cells. Understanding the quiescent state and how cells control the transitions in and out of this state is of fundamental importance. And yet, we know relatively little about it, due to a lack of tools for identifying and studying quiescent cells in their natural setting. The quiescent stateof budding yeast shares many important features with that of higher cells and the cell cycle is fundamentally conserved. As such, the strategies for arresting and maintaining this non-dividing quiescent state are likely to be shared. We propose to identify genes that influence the regulation and longevity of quiescent yeast cells with the goal of providing tools and testable models for defining quiescence in more complex settings. Previous studies of the longevity of yeast in the non-dividing state, or chronological aging, have all involved monitoring the long-term viability of stationary phase cultures, but these cultures are heterogeneous both in age and state. We have shown that wild type yeast grown to stationary phase differentiate into at least three cells types, only one of which bears the properties of quiescent cells. We can track, quantify and purify these quiescent cells. These technical advances allow us to identify mutants and polymorphisms that prevent or promote entry and maintenance of the quiescent state. We can monitor the aging process and identify life-extending pathways in a homogenous population of age-matched quiescent cells. We have observed considerable variation in the yield and longevity of quiescent cells in lab and wild yeast strains. We will take advantage of this variatio and new genomic approaches to identify genes and groups of genes that influence the longevity of quiescent cells or regulate the entry into this state. We have identified wild diploid yeast tha fail to sporulate but enter quiescence very efficiently, and vice versa. This suggests that these are alternative cell fates and, depending on their environment, these diploids have evolved regulatory barriers to specify pathway choice. We will exploit these extreme phenotypes to identify these regulators. It is our hope that discoveries made in budding yeast will offer testabl models for the regulation of these important pathways in metazoan cells.